1. Field of the Invention
This invention relates generally to a circuit for capturing the peak value of an electrical signal and, more particularly, to such circuits disposed in a semiconductor integrated circuit.
2. Description of the Prior Art
Data communications equipment generally rely on a decision circuit to ascertain if the signal (symbol or character) transmitted is one of a plurality of predetermined values, such as a logical "one" or a logical "zero". Accurately establishing the decision threshold (or thresholds) is of critical importance so that error free demodulation occurs.
In burst mode communication systems where "packets" of data may be sent to a receiver from multiple transmitters, the establishment of the thresholds must be done for each received packet and as fast as possible so that a minimum of the information transmitted is demodulated incorrectly. A data "packet" typically includes a preamble, containing no data, which is typically considered as overhead for the communication system. The preamble is used to synchronize the receiver to the packet. The synchronization can occur only after valid demodulation of the preamble occurs. Hence, to minimize the length of the preamble, the time spent establishing a valid threshold should be minimized. Similarly, once the threshold has been established, the threshold should not change appreciably between packets (dead time) yet follow the amplitude changes in the received packets.
To establish the thresholds, a peak detector circuit is used to capture and hold the peak value of an input signal. The peak value of the signal is then subdivided in a predetermined manner to categorize the received signal into one of the plurality of predetermined values.
Similarly, peak detectors may be used in the read channel and head positioning circuitry in mass storage systems. The peak detector must rapidly follow the amplitude of signals from a read head as the head moves from sector to sector or track to track. However, the held peak value of the read data should not change appreciably when the head so moves and no data is being read.
The peak detector may also be used to control the gain of an amplifier to provide a substantially constant output amplitude of a signal amplified by the amplifier. To minimize any overshoot of the amplified signal, the peak value thereof must be ascertained as quickly as possible. Further, the peak detector should hold that value for a predetermined amount of time when the signal is removed while rapidly following the changes in the amplitude of the signal if the signal is still present.